Yes, we do have weather (and climate) in Southern California

October 17, 2012

Surprisingly, there is weather in southern California, a lot of it. And it’s not just sunny, warm heading-to-the-beach weather. Some of its weather can be incredibly hazardous. Actually, the odds that geographers attending the 2013 AAG Annual Meeting in Los Angeles will see sun all week is pretty good. April gets the most hours of sunshine (303.5 hours on average) of any month of the year, after summer’s peak sunny months of July and August. Puny rainfall for the whole month averages only 0.91”, with only 3.2 days on average seeing rain. April 2011 was in fact completely dry, as it has been a half dozen other years. And as far as April temperatures go, it’s much to Goldilocks’ liking: not too hot, not too cold. In April, downtown Los Angeles daytime highs average 72.3° Fahrenheit, while it cools off to 54.2 °F in the early mornings. The thermometer reached a sizzling 106 °F on April 6, 1989, but has never dipped to freezing in the last century.

To really describe L.A. weather and climate you first have to describe Los Angeles. L.A. is a sprawling metropolis within an even larger megalopolis that stretches from Santa Barbara to the north to San Diego to the south and Santa Monica and the Pacific on the west to somewhere beyond San Bernardino to the east. Nineteen million residents live in 190 cities across six counties that make up the L.A. region, covering 38,000 square miles. Several U.S. cities could sit comfortably inside the metropolis without feeling squeezed. The largest concentration of people lies in Los Angeles County, with almost 10 million folks sharing a total area of 4,060 square miles. That includes coastal plains, inland valleys, foothills, mountains of over 10,000 feet, and high deserts. L.A. is also a metropolis of many microclimates. On some April days it could be snowing, raining and sunny all at once within L.A. County.

You could call Los Angeles “Hazards City,” since it is home to practically every meteorological and geological (yes, you may feel a shaking) hazard, besides volcanoes (unless you believe certain movies) and blizzards. The L.A. Basin is actually the “tornado capital of the West” having more tornadoes per area than anywhere west of the Rockies. Unlike the monsters of the Midwest, the L.A. variety of twister is usually smaller and less ferocious, with many originating as waterspouts coming onshore along the L.A. and San Diego county coastline. Mostly EF-0 and EF-1 severity on the Enhanced Fujita Scale, L.A. has had a couple tornadoes ranked as high as EF-3. One rather energetic EF-2 tornado hitch-hiked up the 110 Harbor Freeway, mangling structures along the way before ripping off portions of the Convention Center’s roof on March 1, 1983. During El Niño years, there are markedly more severe weather events than non-El Niño years. The 1997-98 El Niño winter spawned 25 tornadoes in the Golden State. Remarkably, after some 400 recorded tornadoes (since 1959), California has not had one fatality.

Summer heat waves, year round fires and fall/winter Santa Ana winds often follow each other to present another detour from the supposedly typical mild California weather. Hot, dry Santa Ana winds can send temperatures soaring above 100 °F and whip small brush fires into gigantic inferno-like wildfires. The unusually long heat wave of July 16-26, 2006 resulted in over 140 fatalities and agricultural losses running into the billions of dollars. Heat waves have become more frequent and longer in duration, especially in the southern California region.

Hot and dry Santa Ana winds flow down the mountain slopes from the Great Basin towards the southern California coast. Very cold, polar air sweeping south into the Great Basin can intensify pressure gradients resulting in north or northeast winds. The air picks up speed as it pushes through narrow canyon passes (Bernoulli Effect), while warming by compression due to increasing pressure at lower elevations. Often the warmest temperatures are found closest to the coast. While Santa Ana winds have not appreciably increased in recent decades, their impacts through wildfires have increased in both northern and southern California. In 2003, over 280,000 acres of southern California burned in the Cedar’s Fire, while in late August, 2009, the Station Fire, north of Los Angeles, was the largest fire in the state’s history, scorching 160,577 acres (251 sq mi; 650 km2) and killing two firefighters. Santa Anas, also called Santanas or “devil winds,” are so prominent a feature of southern California that they have sometimes been used as a backdrop in literature, such as in the short story, “Red Wind,” by Raymond Chandler (1938).

“There was a desert wind blowing that night. It was one of those hot, dry Santa Anas that come down through the mountain passes and curl your hair and make your nerves jump and your skin itch. On nights like that every booze party ends in a fight. Meek little wives feel the edge of the carving knife and study their husbands’ necks. Anything can happen.”

Although Los Angeles experiences more dry years than wet (6 out of 10 winters are drier than “normal”), the city has a remarkable history of devastating floods and even more deadly droughts. The most memorable flooding in historical times was the famous 1861-62 event, when from December 24, 1861 until January 31, 1862, almost continuous downpours drowned all of California. Much of the Central Valley was an enormous lake, while in Los Angeles rain occurred on 30 consecutive days. Extensive flooding and massive mudslides ravaged property and roads. After the devastating 1861-62 flood, a severe drought hit L.A. County with no significant rain occurring for the next two years. (This put an end to a small cattle and livestock industry.) Another storm on March 2, 1938 was the most destructive and violent of the 20th century. Forty-nine fatalities were associated with the flood and millions of dollars in damages resulted. This flood led to an important decision by the Army Corps of Engineers to channelize the L.A. River for flood control – a decision that reverberates to this day. Most major California flooding occurs with atmospheric rivers (AR) – narrow bands of moisture flowing from the tropics into the west coast. The “Pineapple Express” is one type of AR that originates near the Hawaiian Islands.

When the Pacific Ocean talks, California listens. Changes in the Pacific account for much of southern California’s swings in weather and climate. Take El Niño, for instance. It occurs when sea surface temperatures rise above normal in the eastern and central equatorial Pacific once every three to eight years. The opposite, La Niña, occurs when the same region experiences unusual cooling. It is generally thought, especially by my students, El Niño years are associated with floods in California. Actually, this is not always the case. However, El Niño years do bring more storms, more days of rain and above normal precipitation to southern California, in general, while La Niña years are called “the Diva of Drought” by JPL climatologist, Bill Patzert. In the 1997-98 El Niño, one of the strongest ever recorded, L.A. averaged 230% of normal precipitation over the whole district.

In recent decades Los Angeles has been getting warmer and drier. Is this all global warming? Not if you consider that the city has warmed 5 °F in the last century, while global warming is more like one or two °F. This rapid warming is definitely a result of the intensifying urban heat island (UHI), a phenomenon very apparent in sprawling western cities where the concentrated buildings of the Central Business District (CBD) create an island of warmer temperatures than the surrounding rural areas. Population growth is a major driving force for the increase in the UHI effect, as L.A. has doubled in population since 1950.

So, if you are influenced by how L.A. weather is portrayed in the movies, are you expecting LA Story (sun, sun, sun) or the unrelenting gloom of Bladerunner? In any case, enjoy sunny southern California next April at the AAG Annual Meeting. Or as Mark Twain said, climate lasts a long time and weather only a few days.

Steve LaDochy is professor of Geosciences & Environment at California State University, Los Angeles and Summer Faculty Fellow at the Jet Propulsion Laboratory (JPL), NASA. He is the author of several articles on urban climates, climate variability and change, and air pollution meteorology. In his past life he’s worked with the National Weather Service, air pollution control district, and as a KNBC weather forecaster.